Method of forming a sample image and charged particle beam apparatus

ABSTRACT

An object of the present invention is to provide a sample image forming method and a charged particle beam apparatus which are suitable for realizing suppressing of the view area displacement with high accuracy while the influence of charging due to irradiation of the charged particle beam is being suppressed. 
     In order to attain the above object, the present invention provide a method of forming a sample image by scanning a charged particle beam on a sample and forming an image based on secondary signals emitted from the sample, the method comprising the steps of forming a plurality of composite images by superposing a plurality of images obtained by a plurality of scanning times; and forming a further composite image by correcting positional displacements among the plurality of composite images and superposing the plurality of composite images, and a charged particle beam apparatus for realizing the above method.

TECHNICAL FIELD

The present invention relates to a method of forming a sample image anda charged particle beam apparatus, and particularly to a method offorming a sample image and a charged particle beam apparatus which aresuitable for obtaining a high resolution image in a high magnificationand not influenced by image drift.

BACGROUND ART

In a charged particle beam apparatus typical of which is a scanningelectron microscope, desired information (for example, a sample image)is obtained from a sample by scanning a thinly converged chargedparticle beam on the sample. In such a charged particle beam apparatus,the resolution becomes higher year by year, and the required observationmagnification becomes higher as the resolution becomes higher. As thebeam scanning method for obtaining a sample image, there is a methodwhich obtains a final objective image by adding a plurality of imagesobtained by high speed scanning and a method which obtains a finalobjective image by once of low speed scanning (acquiring time of oneframe image: approximately 40 seconds to 80 seconds). The influence ofthe drift of a view area on the acquired image becomes more serious asthe observation magnification becomes higher. For example, in the methodof acquiring the objective image by adding image signals obtained by thehigh speed scanning pixel by pixel (frame addition), when there is driftcaused by charge-up of the sample during adding the images, theobjective image after adding has blurs in a direction of the driftbecause displaced pixels of the view area are added. Reducing theinfluence of the drift may be obtained by reducing the number of addingframes and shortening the adding time, but this method cannot obtain asufficient S/N ratio.

On the other hand, in the method of acquiring the image by the low speedscanning, when there is drift during acquiring the image, the image isdeformed because the view area flows in a direction of the drift.

A technology is disclosed in Japanese Patent Application Laid-Open No.62-43050. The technology is that a pattern for detecting drift isstored, and a beam irradiating position is corrected by periodicallyacquiring an image of the pattern to detect a displacement between theacquired image and the stored pattern.

A technology is disclosed in Japanese Patent Application Laid-Open No.5-290787. The technology is that two images are acquired based onelectron beam scanning on a specified observed area, and patternmatching is performed in order to specify an amount of displacement anda direction of displacement between the both images, and pixels areadded by moving the pixels by the specified amount of displacement andthe specified direction of displacement.

In the technology disclosed in Japanese Patent Application Laid-Open No.62- 43050, the accuracy of controlling the beam irradiating positionbecomes insufficient when the observation magnification becomes severalhundred thousand times. For example, when an image of 1280×960 pixels istried to be acquired with an observation magnification of 200 thousandtimes, the size of one pixel on the observation view area (on thesample) is approximately 0.5 nm. Measurement and evaluation with ahigher magnification becomes necessary as the scale-down of a measuredobject is progressed. Under such a condition, when the technology isapplied to an apparatus for forming a final image by adding a pluralityof images, image shift (drift) below several nm causes “blurs” in aframe added image.

Although the technology disclosed in Japanese Patent ApplicationLaid-Open No. 62-43050 suppresses the image shift by controlling thescanning position of the electron beam to correct the drift, thecorrecting accuracy of the position by such control is limited toseveral nm to several tens nm. Accordingly, it is almost impossible tocorrect the position (correct the drift) of an image having amagnification the position above several hundred thousand times with apixel level. In addition, there is a problem in that the through-put isdecreased because stabilization of the drift takes a long time.

On the other hand, the technology disclosed in Japanese PatentApplication Laid-Open No. 5-290787 can be appreciated in the point thatthe position between the images can be corrected in the pixel level, butthere is the following problem.

DISCLOSURE OF THE INVENTION

Because an S/N ratio of image data before processing image adding is lowand accordingly the displacement between the images is difficult to bedetected, it is difficult to correct the displacement with highaccuracy. Further, it can be considered that the S/N ratio is improvedby increasing the probe current (the electron beam current) to increasethe amount of secondary electron emission. However, in a case of aneasily charged sample, the displacement between the images acquired atdifferent timing is further increased by movement of the view area ofthe electron beam due to charging, and as the result, it has beendifficult to correct the displacement with high accuracy. Furthermore,in a case where a sample weak against electron beam damage is irradiatedby an electron beam having a large beam current, there is a problem inthat the sample may be broken or evaporated.

An object of the present invention is to provide a sample image formingmethod and a charged particle beam apparatus which are suitable forrealizing suppressing of the view area displacement with high accuracywhile the influence of charging due to irradiation of the chargedparticle beam is being suppressed.

In order to attain the above object, the present invention provide amethod of forming a sample image by scanning a charged particle beam ona sample and forming an image based on secondary signals emitted fromthe sample, the method comprising the steps of forming a plurality ofcomposite images by superposing a plurality of images obtained by aplurality of scanning times; and forming a further composite image bycorrecting positional displacements among the plurality of compositeimages and superposing the plurality of composite images, and a chargedparticle beam apparatus for realizing the above method.

As described above, since positional displacements can be detected amongimages having a sufficient S/N ratio without increasing beam current byforming composite images and then correcting the positionaldisplacements, “blurs” of an image at adding the frames can besuppressed because the positional displacements are corrected with highaccuracy. The other objects of the present invention and the otherdetailed construction of the present invention will be described in thesection “DESCRIPTION OF THE PREFERRED EMBODIMENTS” in the presentspecification.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a scanning electron microscope forexplaining an embodiment in accordance with the present invention.

FIG. 2 is a flowchart showing the processing of reconstructing anobjective image by correcting positional displacements of a plurality ofacquired images.

FIGS. 3( a) and 3(b) are photographs showing an image obtained by simplyadding images and an image obtained by correcting positionaldisplacements after acquiring a plurality of images and then adding thepositional displacement corrected images.

FIG. 4 is a flowchart showing the processing combining of the processingof correcting the drift by controlling the beam irradiating position orthe sample position and the processing of correcting the positionaldisplacements of a plurality of acquired images and then adding theplurality of images of which the positional displacements are corrected.

FIG. 5 is a conceptual view showing the process of adding a plurality ofimages while positional displacements among the plurality of images arebeing corrected.

FIG. 6 is a flowchart showing the processing of restoring deformation ofan image acquired by slow scanning, the deformation being caused by thedrift.

FIG. 7 is a conceptual view showing the process of restoring tdeformation of the image acquired by slow scanning, the deformationbeing caused by the drift.

FIG. 8 is a conceptual view showing the process of correcting thepositional displacements among a plurality of profiles obtained throughline scanning and then adding the plurality of images of which thepositional displacements are corrected.

FIG. 9 is a graph showing an example of estimating the beam damage frommeasured length values in a plurality of acquired images to calculate ameasured value of length which is not influenced by beam damage.

FIG. 10 is a view showing an example in which a plurality of images eachhaving a region wider than a view area of an objective image areacquired, and after adding the images of which the view areadisplacements among the images are corrected, the region of theobjective view area in the central portion is cut out.

FIG. 11 is a view showing an example in which an image having a detectedabnormality is removed out of a plurality of acquired images, and thenthe images excluded the abnormal image are added after correcting theview area displacement.

FIG. 12 is a view showing an example in which view area displacementsamong a plurality of images acquired by detecting a plurality of imagesignals at a time are corrected, and then the images are added.

FIG. 13 is a view showing an example in which positional displacementsof a plurality of images are corrected only in a specified direction,and then the images are added.

FIG. 14 is a view explaining a method of detecting the amount ofpositional displacement and adding the corrected images.

FIG. 15 is a view explaining another method of detecting the amount ofpositional displacement and adding the corrected images.

FIG. 16 is a view showing an example of a GUI page displayed on an imagedisplaying unit.

FIG. 17 is a view showing an example of an electron detecting system inan embodiment of a charged particle beam apparatus in accordance withthe present invention.

FIG. 18 is a view showing another example of an electron detectingsystem in an embodiment of a charged particle beam apparatus inaccordance with the present invention.

FIG. 19 is a view showing an example of a GUI page displayed on an imagedisplaying unit.

FIGS. 20( a) and 20(b) are views for explaining the principle ofEmbodiment 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described below, referringto the accompanied drawings.

FIG. 1 is a block diagram showing an embodiment of a scanning electronmicroscope in accordance with the present invention. A voltage isapplied between a cathode 1 and a first anode 2 by a high voltagecontrol power source 20 controlled by a computer 40 to extract a primaryelectron beam 4 with a preset emission current from the cathode 1. Anacceleration voltage is applied between the cathode 1 and a second anode3 by the high voltage control power source 20 controlled by the computer40, and the primary electron beam 1 emitted from the cathode 1 isaccelerated and travels to a lens system in the rear stage.

The primary electron beam 4 is focused by a focusing lens 5 controlledby a lens control power source 21. Then, after unnecessary regions ofthe primary electron beam are removed by an aperture plate 8, theprimary electron beam 4 is focused on a sample 10 as a very small spotby a focusing lens 6 controlled by a lens control power source 22 and anobjective lens 7 controlled by an objective lens control power source23. The objective lens 7 may be of various type such as an in-lens type,an out-lens type, a snorkel type (a semi-in-lens type) etc. Further,each of the lenses may be constructed of an electrostatic lens which iscomposed of a plurality of electrodes.

The primary electron beam 4 is two-dimensionally (in X-Y directions)scanned on the sample 10 by a scanning coil 9. Current is supplied tothe scanning coil 9 from a scanning coil control power source. Secondarysignals 12 generated from the sample 10 by irradiation of the primaryelectron beam travel to the upper portion of the objective lens 7, andthen are separated from the primary electrons by a secondary signalseparation orthogonally-crossing electro-magnetic field generator 11 tobe detected by a second signal detector 13. The signals detected by thesecondary signal detector 13 are amplified by a signal amplifier 14, andthen transmitted to an image memory 25 and displayed on an image displayunit 26 as a sample image. The secondary signal detector 13 may be adetector for detecting secondary electrons or reflected electrons, or adetector for detecting light or X-rays.

An address signal corresponding to a memory area of the image memory 25is generated in a computer 40, and converted to an analogue signal, andthan supplied to the scanning coil 9 though the scanning coil controlpower source 24. The address signal in X-direction is a digital signalrepeating, for example, 0 to 512 in a case where the image memory 25 is512×512 pixels, and the address signal in Y-direction is a digitalsignal repeating 0 to 512 which is added by 1 when the address signal inX-direction reaches 512 from 0. The signals are converted to theanalogue signals.

Since the address of the image memory 25 corresponds to the address ofthe reflection signal for scanning the primary electron beam, atwo-dimensional image of the deflection region of the primary electronbeam by the scanning coil 9 is recorded in the image memory 25. Thesignals in the image memory 25 can be sequentially and successively readout using a read-out address generating circuit (not shown) synchronizedby a read-out clock. The signal read-out corresponding to the address isconverted to an analogue signal, and becomes a brightness modulatedsignal for the image display unit 26.

The image memory 25 has a function for superposing (adding) the images(image data items) in order to improve the S/N ratio and then storingthe composite image. For example, by superposing images obtained by 8times of two-dimensional scanning and then storing the composite image,one frame of complete image is formed. That is, a final image is formedby adding images which are formed by once or more times of X-Y scanning.Number of images (number of adding frames) for forming one frame of thecomplete image may be arbitrarily set, and an appropriate number is setin taking into consideration conditions such as secondary electrongenerating efficiency and so on. Further, by superposing a plurality offrames each of which is formed by adding the plurality of images, afinally desired image may be formed. By executing blanking of theprimary electron beam at the time when a desired number of image framesare stored or after the time, information input to the image memory maybe interrupted.

Further, in a case where the number of adding frames is set to 8, it ispossible to provide such a sequence that the first frame of image may bedeleted when a ninth frame of image is input so that 8 frames of imageremain as the result. Otherwise, it is possible to perform weightedaddition averaging. That is, when a ninth frame of image is input, anadded image stored in the image memory is multiplied by ⅞ and then theninth frame of image is added to the added image after being multipliedby ⅞.

A two-stage deflecting coil 51 (an image shift deflector) is arranged ata position the same as that of the scanning coil 9, and thereby, theposition of the primary electron beam 4 (the observed area) on thesample 10 can be two-dimensionally controlled. The deflecting coil 51 iscontrolled by a deflecting coil control power source 31.

A stage 15 can move the sample 10 at least in 2 directions (X-directionand Y-direction) on a plane normal to the primary electron beam.

From an input unit 42, an image acquiring condition (scanning speed,number of adding frames of image) and a method of correcting view areacan be specified, and outputting and storing of the images can also bespecified.

Further, the embodiment of the apparatus in accordance with the presentinvention comprises a function for forming a line profile based ondetected secondary electrons or detected reflected electrons. The lineprofile is formed based on an amount of detected electrons when theprimary electron beam is one-dimensionally or two-dimensionally scannedor based on brightness information of the sample image, and the obtainedline profile is used for dimension measurement of a pattern formed, forexample, on a semiconductor wafer. The embodiment of the apparatus inaccordance with the present invention may further comprise an interface41 for transmitting image data to an external unit or the like, and arecording unit 27 for storing image data to an appropriate memorymedium.

In the explanation of FIG. 1, the control unit is described as a unitintegrated with the scanning electron microscope or the like, but it is,of course, not limited to such a unit. A control processor separatelyprovided from the scanning electron microscope may be used to executethe processing as described below. At that time, a transmitting mediumfor transmitting signals from the control processor to the scanningelectron microscope and input and output terminals for inputting andoutputting the transmitted signals through the transmitting medium arenecessary.

Further, it is possible that a program for executing the processing tobe described below is registered in a memory medium, and the program isexecuted by the control processor for supplying necessary signals to thescanning electron microscope having an image memory. That is, theembodiments of the present invention to be described below also hold asthe invention of program which can be employed to a charged particlebeam apparatus such as a scanning electron microscope having an imageprocessor.

EMBODIMENT 1

In an embodiment of a method of improving an S/N ratio by adding TVscanned images, the processing flow of FIG. 2 will be described below indetail. FIG. 5 is a view schematically showing the processing of FIG. 2.

First Step (S2001):

Number N0 of adding frames for each acquired image and number N1 ofacquired image sheets are specified. At that time, total number ofadding frames of the final image is N0×N1. In general, by setting thenumber N0 to 2 frames to 8 frames and the number N1 to 10 sheets to 50sheets, a necessary S/N ratio can be obtained depending on the purpose.In a case where each of image is acquired with slow scanning slightlyslower than the TV scanning, the number N0 may be set to 1 frame. In acase of TV scanning of interlace type, the number N0 can be set to 2. Inregard to the condition setting, it is preferable that the plurality ofsample images are formed by fixing the optical conditions (a focusingcondition of the electron beam and a scanning condition) in order tomake detection of positional displacement easy.

Second Step (S2002):

As starting of acquiring image is instructed from the input unit 42, N1sheets of images of frame adding number N0 (F1, F2, . . . , FN1) in thesame view area are successively acquired.

Third Step (S2003):

F1 is set to a memory area of the objective image F0.

Fourth Step (S2004):

A sharpened image F0 a is produced from the objective image F0. As thesharpening processing, a technique using an image filter for emphasizingedges in the image may be used.

Fifth Step (S2005):

A sharpened image F2 a is produced from the image F2.

Sixth Step (S2006):

A positional displacement between the sharpened image F2 a of F2 and thesharpened image F0 a is detected. Calculation processing such as imagecorrelation may be applied to the detection of the positionaldisplacement. However, of course, the present invention is not limitedto the above, and all the image processing methods capable of detectingthe positional displacement are applicable.

Seventh Step (S2007):

Pixels of the original image F2 is shifted by the amount of thedisplacement of view area detected in the Sixth Step and added to theimage of F0, and then the formed image is returned as the objectiveimage F0 again.

Eighth Step (S2008):

By repeating the Fourth Step to the Sixth Step Substituting F3 for F2,the adding processing with the correction of positional displacement isexecuted to all the N1 sheets of images.

In the present embodiment, the finally obtained image is an image formedby adding N0×N1 frames, but the image is blurred by the drift only whenN0 frames are added. Therefore, the blur of the image by the drift isreduced to 1/N1 compared with the case of directly adding N0×N1 frames.By employing such a sequence, it is possible to remove the positionaldisplacement in a direction on the two-dimensional image plane betweenimages acquired at different timing due to charge-up on the sample, andaccordingly, image blurs of the image can be suppressed or eliminated.

FIGS. 3A and 3B show an example of a result obtained by this embodiment.FIG. 3( a) is an image obtained through commonly adding the frames(1280×960 pixels, 200 thousand times of magnification), and drifts areaccumulated during adding the images to form conspicuous “blur” in thefinal image. FIG. 3( b) is an image obtained by acquiring 10 sheets ofimages having frame adding number 1/10 times as small as the frameadding number of FIG. 3( a), and adding these 10 sheets of the imageswhile the positional displacements are being corrected. In FIG. 3( b),though the total frame adding number of images is the same as that ofFIG. 3( a), the “blur” in the final image caused by the drift is alsoreduced to 1/10 times as small as that in FIG. 3( a) because only thedrift accumulated in each of the added images becomes “blurred” in thefinal image and the acquiring time for each image is 1/10 times as shortas that in FIG. 3( a).

Since the amount of drift changes depending on the kind of the sample,the optical condition and so on, it is preferable that N0 and N1 are setcorresponding to the S/N ratio. Since number of scanning times (numberof images) required for securing a required S/N ratio is determinedbased on the quality of obtained image and the efficiency of generatingsecondary electrons, N0 and N1 may be determined in taking the degree ofdrift into consideration. Further, it is also possible to construct thesequence that by inputting a parameter expressing conditions of thesample (easiness of charge-up etc) and at least one of total addingnumber, number of adding frames (N0) and number of acquired images (N1),the other two parameters are determined. According to such aconstruction, the apparatus condition can be easily set only byinputting specification necessary for observation.

In the present embodiment, although the positional displacement betweenthe frame-added images is corrected, the present invention is notlimited to the above. Correction of the positional displacement may beexecuted by the unit of an arbitrary number of frames or by the unit ofarbitrary number of acquired sheets. At that time, unless an image to becompared with for detecting the positional displacement has an S/N ratiolarger than a certain value, the drift detecting accuracy will bedecreased. Therefore, it is preferable that number of images necessaryfor securing a desired S/N ratio is set as the frame adding number (N0),and then number of acquired image sheets (N1) for obtaining a necessaryS/N ratio for the final sample image is set.

In the present invention, the image may be stored in the image memory 25after correcting the positional displacement. Otherwise, by preparing aframe memory corresponding to (frame adding number)×(acquired images),the positional displacement among sample images may be corrected whenthe sample image is displayed, or when the sample image is transferredto an external image memory element, or before the sample image istransferred to the external image memory element. Otherwise, thepositional displacement among sample images may be corrected in theexternal image memory element.

By preparing at least an image memory for storing a composite image, animage memory for storing images before executing superposing processingand an image memory for storing an image to be acquired, images acquiredone after another by the electron beam scanning can be successivelysuperposed.

In the present embodiment, in order to make the setting of N0 and N1 forspecified samples easier, the system may be constructed in such that areference image for each combination of N0 and N1 is stored, and thereference image can be read out at setting N0 and N1. By doing so, anoperator can set appropriate N0 and N1 by referring to the referenceimage.

It is preferable that when drift is fast, the number of displacementcorrections is increased by decreasing the number of frames N0, and thatwhen drift is not so fast, the number of frames N0 is increased in orderto improve the quality of the image to be compared with. For example, itis preferable that as a means for appropriately setting the numbers N0and N1, a means for adjusting N0 and N1 stepwise is provided. In a casewhere the total adding frame is set to 50, the combinations of N0 and N1are 1×50, 2×25, 5×10, 10×5, 25×2, and 50×1. However, by providing ameans for adjusting the combination and a means for displaying anactually added image, the operator can set the appropriate N0 and N1from the superposed image without detailed knowledge of the technologyin regard to the present invention.

By providing the adjusting means described above, not only in the caseof correcting the displacement, but also in a case where the quality ofimage is changed by changing the combination of N0 and N1, anappropriate combination of N0 and N1 can be easily selected.

Further, the same effect can be attained by providing a means foradjusting the degree of displacement correction which sets N1 to alarger value when “the degree of displacement correction is large” isselected, and sets N0 to a smaller value when “the degree ofdisplacement correction is small” is selected.

EMBODIMENT 2

A processing flow of FIG. 4 will be described below in detail.

First Step (S4001):

Number N0 of adding frames for each acquired image and number N1 ofacquired image sheets are set.

Second Step (S4002):

Two sheets of images of the frame adding number N0 are successivelyacquired.

Third Step (S4003):

Sharpened images are generated from the acquired two sheets of images,and a positional displacement between the sharpened images iscalculated.

Therein, when the amount of this displacement exceeds a preset allowablevalue, each of the images before correcting the position and being addedconspicuously includes “blurs” due to drift. Therefore, the processingis stopped, and a display function may notify the operator that thedrift is too large.

Fourth Step (S4004):

The two sheet of images after correcting the positional displacement areadded to each other, and the added image is registered as F0.

Fifth Step (S4005):

The view area is moved in a direction canceling the positionaldisplacement obtained in the process of S4003. Therein, as the shiftingmeans, each of a method of using an electric view-area shifting means(an image shift deflector) and a method of using a stage is availabledepending on the amount of shifting. In general, when the amount ofshifting is small, both of the image shift deflector and the stage areused. When the amount of shifting is large, the stage is used or theimage shift deflector is used if necessary. By canceling thedisplacement of the view area using image shift deflector and the stage,the displacement between the images can be compressed even if there is acomparatively large drift. Therefore, it is possible to solve theproblem that an effective view area (an area where view areas of imagesare overlapped with one another) after correcting the positionaldisplacement by the image processing becomes narrow.

Sixth Step (S4006):

The next image is acquired.

Seventh Step (S4007):

By forming a sharpened image of the acquired image and a sharpened imageof F0, a positional displacement between the sharpened images iscalculated.

Eighth Step (S4008):

The image F0 and the image acquired in S4006 are added by correcting thepositional displacement between the images, and the added image is newlyset as F0.

Ninth Step (S4009):

The view area is moved in a direction canceling the positionaldisplacement obtained in the process of S4008.

Tenth Step (S4010):

By repeating the process S4006 to the process S4009, N1 sheets of imagesare obtained, and the obtained images are added.

According to the above construction, a large drift component can becorrected by the stage and the beam deflection, and very small drift ofpixel level can be corrected at adding the images. Therefore, a highresolution image can be obtained by effectively correcting even acomparatively large drift.

On the other hand, in order to minimize the effect of drift, it isnecessary to minimize the acquiring time of each of the images forcorrecting the positional displacements and then being added to thelimit. However, the limit is determined by the S/N ratio of the imagesnecessary for detecting the positional displacement. Therefore, if theamount of the drift exceeds a certain value, each of the images itselffor detecting the positional displacement becomes blurred due to drift.When an amount of drift causing such a result is detected, a means fordisplaying that a high resolution image is difficult to be acquired orfor stopping the measurement may be provided. By doing so, it ispossible to solve a problem of uselessly operating the apparatus under astate that acquiring of the sample images is clearly difficult.

EMBODIMENT 3

A processing flow of FIG. 6 will be described below in detail.

First Step (S6001):

A first image F1 for detecting drift is acquired.

Second Step (S6002):

An objective image F0 is acquired under an appropriate slow scanningcondition. By acquiring the image under such a slow scanning condition,a high contrast image can be obtained because secondary electrons can begenerated more compared to the case of fast scanning.

Third Step (S6003):

A second image F2 for detecting drift is acquired.

Fourth Step (S6004):

A displacement ΔF (ΔFx, ΔFy) between the images F1 and F2 is detected.

Fifth Step (S6005):

Amounts of deformations in the horizontal direction and the verticaldirection of the objective image are calculated from the amount of imagedisplacement ΔF.

Sixth Step (S6006):

A new image F0′ is formed by deforming the objective image F0.

Here, the processing of Fifth Step will be described below in detail,referring to FIG. 7.

Letting the amount of displacement between the drift detecting images F1and F2 acquired into the image memory be ΔF (ΔFx, ΔFy), and a timedifference between acquiring the image F1 and acquiring the image F2 beΔT, drift speeds (Vx, Vy) in X-direction and Y-direction can becalculated by the following equations.Vx=ΔFx/ΔT,Vy 32 ΔFy/ΔTOn the other hand, letting an acquiring time of the objective image F0be T0, a displacement of view area of the image F0 generated during thetime period from starting scanning to ending scanning can be expressedas follows.X-direction ΔF0x=Vx×T0Y-direction ΔF0y=Vy×T0

Therefore, as shown in FIG. 7, by deforming the objective image F0 inthe image memory by F0 x (Y-direction) and ΔF0 y (X-direction) towardthe directions of correcting the drift, it is possible to reproduce theestimated image F0′ which would be obtained if the drift did not occur.

In the present embodiment, the images F1 and F2 for detecting drift areacquired before and after acquiring the objective image F0,respectively, but, of course, the present invention is not limited tothe above. The images F1 and F2 may be successively acquired beforeacquiring the image F0 or after acquiring the image F0. In that case,the deformation is estimated from the displacement between the images F1and F2 at the time when the image F0 is acquired, and then the estimatedimage F0′ can be reproduced.

Each of the images F1, F2 and F0 is stored in the image memory, and theimages F1 and F2 are read out based on judgment on necessity of imagereproduction, and then the reproduction processing is performed.

According to the construction described above, the shape of an observedobject deformed by drift can be accurately known.

Particularly, in the case of slow scanning, the electron beam is beingirradiated on the sample, and accordingly deformation of the sampleimage due to charging of the sample becomes large. Therefore,application of the technology of the present embodiment is veryeffective in slow scanning. Further, although the present embodiment hasbeen described on the case of two images for correcting drift and oneimage to be corrected, the present invention is not limited to the aboveand arbitrary number of images may be used.

Further, in order that the operator judges the necessity of imagereproduction, in the apparatus of the present embodiment, option buttonsfor selecting necessity of image reproduction are provided on agraphical user interface (GUI), as shown in FIG. 16. Although FIG. 16shows the example of performing selection using a pointing device or thelike on the image display unit, the present invention is not limited tothis method. Setting may be performed using another well-known inputsetting means.

In an observation using an electron beam apparatus such as a scanningelectron microscope, there is a need that the sample image is highlyaccurately formed. On the other hand, there is also a need that damageof the sample is reduced by suppressing irradiation of the electron beamas low as possible. In the case of the embodiment of the apparatus inaccordance with the present invention, the sample image can be formedwith high accuracy if the deformation of the sample due to drift can besuppressed, but the electron beam scanning for acquiring at least theimages F1 and F2 is farther required, which is different from the caseof simply forming the image F0. That is, since the scanning time of theelectron beam is increased, possibility of the sample damage caused bythe electron beam irradiation is increased.

By providing the option described above, the operator can selectnecessity of the reproduction taking a status of the observed object orthe condition for forming an appropriate sample image intoconsideration, and can form the sample image which the operator desires.

Further, by making a graph on what extent the deformation is correctedand registering the graph, the information can be used for setting thescanning speed and for judgment of necessity of drift correction.Further, by storing and displaying number of scanning times of theelectron beam and amount of correction and the amount of deformationversus the irradiation time, it is possible to know the sample image isdeformed by how long the electron beam is scanned.

EMBODIMENT 4

An embodiment of applying the drift correction technology to addition ofline profiles will be described below, referring to FIG. 8.

In general, measurement of dimension of a pattern on a wafer uses asignal distribution (a line profile) which is obtained when an electronbeam is line-scanned on a pattern of a measured object. In a case wherethe sample is an insulator, high speed scanning is performed in order toprevent disturbance caused by charging. Therefore, since a signalobtained by once of line scanning is bad in the S/N ratio, it isdifficult to perform highly reproducible measurement. Accordingly, ingeneral, signal distributions obtained from several times of scanningare added to form a profile of the measured object. At that time, ifdrift occurs in the direction of line scanning, the added line profilebecomes dull, and accordingly the accuracy of measurement is decreased.

Therefore, each of the profiles obtained by plural times of linescanning is stored, and positional correction of the profiles isperformed so that the correlation among the profiles becomes highest,and then the profiles are added. In this case, a signal acquired by onceof scanning may be used as each of the profiles before addition.However, when the scanning speed is high, the signal acquired by once ofscanning is too bad in the S/N ratio. Therefore, signals obtained byminimum number of scanning times within a range capable of correlatingamong the profiles are simply added, and the added signal may be used aseach of the profiles before addition. By this method, the problem ofdullness of the profile is improved even if there is drift, and aprofile having a high S/N ratio can be produced. Therefore, it ispossible to perform highly reproducible measurement.

Further, the apparatus may be constructed in such that the setting pagedescribed in FIG. 16 is also used for selecting the necessity ofpositional correction. A semiconductor inspection apparatus or the likemay be constructed in such that reliability of the measurement can bechecked later by sounding an alarm or setting a flag to the measurementwhen an amount of positional correction exceeds a threshold and clearlyincreases. Therein, in a case where an amount of correction, a addedprofiler and profiles before adding are stored, or in a case where theline profile is formed based on a two-dimensional scanned image, theapparatus may be constructed in such that the added image or the imagesbefore adding are stored and displayed on an image display unit later.

According to the construction described above, when a measured object iserroneously measured in a repetitive pattern or the like where patternsof the same type are adjacently arranged, the erroneous measurement canbe easily checked.

Particularly, in the case where a line profile is formed throughone-dimensional scanning, the measurement can be rapidly performedcompared to the case of two-dimensional scanning. However, it isimpossible to check the accuracy of measurement by referring the sampleimage. In the present embodiment, the line profile can be formed withhigh accuracy even in the case of one-dimensional scanning by which themeasurement can be rapidly performed. For example, even in an apparatusmeasuring length of a pattern based on a line profile, the length can bemeasured with high accuracy based on the line profile formed with highaccuracy.

In a case where a pattern width of a line pattern having roughness ismeasured, the measuring length range is expanded toward directionsperpendicular to the direction of measuring length, and measurement oflength based in the line profile is performed using a plurality ofdifferent positions within the measuring length range. Then, theplurality of obtained measured length values are averaged, or thedispersion values of roughness are measured based on the plurality ofmeasured length values obtained within the expanded measuring lengthrange. The present embodiment is also applicable to this case.

For example, by performing addition of line profile with theabove-mentioned positional correction for each of the plurality of thelength measuring positions, and then by measuring the average value orthe dispersion value of roughness, these resultant values can beobtained with high accuracy.

Even in a case where the line profiles are displaced depending on thelength measuring position due to charge-up or the like, the added lineprofile can be appropriately formed and the length can be accuratelymeasured by performing positional correction, using one reference lineprofile, to the line profiles in the other positions, not by performingpositional correction for each of the plurality of the length measuringpositions. According to the construction described above, reliabilityevaluation of electric property of a semiconductor element pattern canbe easily realized regardless of existence of roughness.

Further, in the case of measuring lengths of a plurality of positions,when a measured length value of one of the positions is extremelydifferent from the measured length values of the other of the positions,there is a possibility that a part of the line pattern is extremelythinned, or that a failure of the length measurement occurs. In such acase, the apparatus may be constructed in such that an error message isoutput or that the measured results such as the sample image and theline profile are registered together with the measuring conditions so asto check the results later by read out the data.

EMBODIMENT 5

FIG. 9 is a graph for explaining an example of estimating an accuratedimension from time-varying pattern dimensions by repeating measurementof the same pattern plural times. The object to be measured using anelectron beam is damaged to be shrunk or evaporated depending on thematerial by irradiation of the electron beam. In such a case, since thepattern dimension is decreased as the amount of beam irradiationincreases, the measurement itself is an error cause.

In order to evaluate the correct dimension by estimating the errorcaused by the measurement itself, the same pattern is measured pluraltimes. Since the amount of beam irradiation is increased in proportionto number of measuring times, deformation of the pattern is alsoincreased as the number of measuring times is increased. Therefore, thepattern dimension before irradiating the beam or before shrinking atstarting the beam irradiation can be estimated by obtaining therelationship between the number of measuring times (in proportion to theamount of beam irradiation) and the dimension measured value. In theembodiment of the apparatus in accordance with the present invention, asequence for automatically executing the above-described dimensionestimation is installed.

The apparatus may be constructed in such that in order to judge laterwhether or not the dimension estimation is correctly performed, a tablegraphing number of measuring times versus measured value is stored, andthen output to the display unit or an external output unit. For example,in a case where an observed object is shrunk and at the same time driftalso occurs, the dimension estimation may be not appropriately performedby influence of the drift, the operator can check by referring to theabove-described graph whether or not the dimension estimation iscorrect. By storing a sample image obtained at that time correspondingto the stored graph, the correctness of the dimension estimation can bechecked referring to the sample image.

The apparatus may be constructed in such that when the above-describedgraph records an abnormal trend, the graph is selectively stored or apreset flag is set. For example, when an abnormal change is observed ina graph expressing the trend of dimension change, something may occur inthe electron beam apparatus at that time, and accordingly the dimensionmeasurement may be not correctly performed. If the apparatus isconstructed so that the graph or the sample image can be selectivelychecked at that time, the operator can efficiently check the correctnessof the dimension estimation without performing useless check.

Although the abscissa of the graph expresses “number of measuring times”in the present embodiment, the abscissa may express another parametersuch as “number of scanning times” or “time”. The ordinate is notlimited to express “measured value” either, and the ordinate may expressa ratio of a measured value to a normal value (a design value).

By forming a dummy pattern having a condition equivalent to a measuredobject pattern at a position near the measured object pattern when thepresent embodiment is applied to an apparatus for measuring length of asemiconductor pattern, measurement of length can be accurately performedwithout shrinking the pattern which affect operation of thesemiconductor element.

EMBODIMENT 6

FIG. 10 is a view showing an example in which images, each of which hasa number of pixels larger than the number of pixels of an objectiveimage, are acquired, and displacements among the acquired images arecorrected. The present embodiment shows a case where the number ofpixels of the objective image is, for example, 512×512 pixels. In thisexample, the number of pixels of the acquired image is 1024×1024 pixels.When the acquired images are added by correcting the positionaldisplacements, there appears a region which cannot be used as theobjective image due to displacement among the images. In the presentembodiment, the images each having a region wider than the number ofpixels of the objective image are acquired in advance, and a region of512×512 pixels in the central portion is cut out after adding theacquired images to obtain the final objective image.

Since such slightly larger images are acquired, as described above, itdoes not occur that the peripheral portion of the final image is lost bybeing cut off when drift occurs.

EMBODIMENT 7

FIG. 11 shows an embodiment in which images are added by removing anabnormal image. In a case where an abnormally displaced image or anabnormally blurred image is formed by a sporadic disturbance duringacquiring a plurality of images, or in a case where images acquiredafter acquiring a specific image show abnormal contrast due to chargeduring irradiating the beam, the abnormal image can be removed from theoriginal images to be added by detecting the abnormality through imageprocessing of these images. In regard to displacement, the abnormalitycan be detected by presetting an amount of displacement of view area tobe judged as abnormal. In regard to blur, the abnormal image can beremoved by executing image differential processing or the like, andsetting a threshold to be judged as abnormal. In regard to the abnormalcontrast, the abnormal image can be removed by judging on a histogram orby judging on abnormal decrease in the value of correlation with anotherimage after correcting view area. By removing the abnormal informationas described above, high resolution image can be stably acquired even ifan unexpected cause occurs.

Although the image judged to be abnormal can be removed, in order tosearch the cause of abnormality later, the image judged to be abnormalis stored in the image memory together with the optical conditions(acceleration voltage of the electron source, emission current and soon) at the time when the abnormality is recognized, or before and afterthe time when the image is judged to be abnormal. According to theconstruction described above, it is easy to check what reason theabnormal image is produced by. For example, if the timing that overcurrent flows to the cathode of the electron source agrees with thetiming that the abnormal image is produced, the cause exists in theelectron source, which can be used as an index of replacing of theelectron source.

Changes of current and voltage applied to the optical element such asthe extracting electrode, the acceleration electrode or the scanningcoil of the electron microscope are displayed by a time chart, and thetiming that the abnormality occurs is superposed on the time chart. Bydoing so, the operator can visually specify the cause.

The abnormal frame removing technology explained by the presentembodiment can be applied to the line profile addition explained inEmbodiment 4.

Although the example of mainly automatically removing the abnormal imagehas been described in the present embodiment, the present invention isnot limited to the above. For example, it is possible to provide afunction that images before adding are displayed on the image displayunit, and an image judged to be abnormal by the operator can beselectively removed. Therein, if the apparatus is constructed in suchthat some of images can be selected using a pointing device or the likefrom the plurality of images before adding arranged and displayed on theimage display unit, the operator can be visually select images to beremoved from the plurality of images before adding. The apparatus may beconstructed in such that not only the images before adding aredisplayed, but also the plurality of added images are displayed in orderto ascertain abnormal images using the images having a some degree ofS/N ratio.

EMBODIMENT 8

FIG. 12 shows an embodiment in which view area displacements among aplurality of images acquired by a plurality of image signals arecorrected, and then the images are added. For example, when an addedimage using reflected electron signal is tried to be acquired, a lot offrames must be acquired for each of the original images because theamount of the reflected electron signal is generally little. The reasonis that if an original image is formed by acquiring a small number ofimage frames acquired by the small signal amount, the view areadisplacement among the images can not detected because the S/N ratio ofthe original image is extremely decreased. On the other hand, if numberof the original image frames is increased, the original image itself isblurred due to drift because the time acquiring the original imagesbecomes long. In the present embodiment, the original images areacquired by the reflected electron signal, and at the same time originalimages having a good S/N ratio are acquired using secondary electronsignal, and view area displacement among the original images acquired bythe secondary electron signal is detected, and then the amount of thedetected view area displacement is applied to the view area displacementamong the plurality of images obtained by the reflected electron signal.

Since the secondary electron image and the reflected electron image areacquired at the same time, the view area of the reflected electron imagecompletely agrees with the corresponding secondary electron image.Therefore, the view area displacement of the original reflected electronimages having a bad S/N ratio can be accurately corrected through themethod of the present embodiment. Since the secondary electron signalimage having a high S/N ratio is used as the image for detecting theview area displacement, number of frames composing the original imagecan be minimized. Therefore, the original image itself is not blurred bydrift. As examples of signal having a bad S/N ratio, there are, forexample, X-ray signal and sample absorption current. The embodiment ofthe present invention can be applied to various kinds of signals.Particularly, in a case where an element distribution (an X-ray image)of a thin film sample is acquired with high resolution, the secondaryelectron signal in the present embodiment may be replaced bytransmission electron signal. In general, occurrence of X-raysscattering inside a sample can be prevented by making the sample into afoil having a thickness of several tens nm, and accordingly a highresolution element distribution image can be obtained.

As the detection system for detecting secondary electrons and reflectedelectron at the same time, a construction shown in FIG. 17 isconsidered. According to this construction, two kinds of electrons(reflected electrons 1706, secondary electrons 1707) emitted from asample 1705 can be detected at the same time using a reflected electrondetector 1703 and a secondary electron detector 1704 arranged at anupper position and at a lower position of an objective lens 1702 forfocusing a primary electron beam 1701, respectively.

Further, secondary electrons and reflected electrons can be detectedtogether using a detection system shown in FIG. 18. In the case of theconstruction of FIG. 18, secondary electrons and reflected electrons1803 are accelerated by a retarding voltage 1802 applied to a sample1801, and collide against a secondary electron converting electrode 1805arranged above an objective lens 1804. At the collision, the acceleratedsecondary electrons and the accelerated reflected electrons 1803 producesecondary electrons 1806, and the secondary electrons 1806 are attractedto a secondary electron detector 1807 to be detected.

An energy filter 1808 is applied with an energy filter voltage 1809which is equal to or slightly higher than the retarding voltage 1802applied to the sample. By applying such a voltage, only the reflectedelectrons are selectively pass through the energy filter 1808.

In the construction described above, the secondary electrons and thereflected electrons are alternatively acquired by switching the voltageof the energy filter 1809 on-off or strong-weak every acquiring ofpredetermined number of two-dimensional image frames. Then, thepositional displacement is detected using the secondary electron images,and the positional displacement of the reflected electron image iscorrected using the detected positional displacement information, andthen the reflected electron image is stored in the image memory. Bydoing so, in the scanning electron microscope employing the retardingtechnology, the reflected electron image without blur can be obtained.Although the reflected electrons and the reflected electrons are clearlyseparated in the present embodiment, the present invention is notlimited to the above. The amount of electrons detected by the secondaryelectron detector 1809 may be increased by applying an energy filtervoltage 1809 lower than the retarding voltage 1802 to the energy filter1808. Since most of the electrons emitted from the sample have energysmaller than 50 eV, number of frames composing the original image can beminimized by using electrons having energy smaller than 50 eV for theimages for detecting the view area displacement. The applied voltage tothe energy filter 1808 may be changed depending on the purpose ofanalysis.

The reflected electron detector and the secondary electron detector arenot limited to those described in the present embodiment, but varioustypes of detectors may be employed. Although the X-ray detector has notbeen illustrated, all of the existing X-ray detectors are applicable.

EMBODIMENT 9

FIG. 13 shows an embodiment in which positional displacements of aplurality of acquired images are corrected only in a specified directionon a sample surface, and then the images are added. In a case where animage has a pattern only in a specified direction in the image,positional displacement in a direction perpendicular to the pattern canbe detected with high accuracy, but accuracy of detecting positionaldisplacement in a direction parallel to the pattern is extremely low. Inregard to such an image, by adjusting view areas only in the directionperpendicular to the pattern and adding the images, the error in theview area adjusting can be reduced. The direction of the pattern can bespecified by analysis of frequency components of the image or lineprofiling of the image by binarization.

In an apparatus for measuring the line width of a pattern on asemiconductor wafer, the accuracy of a result of the length measurementcan be maintained even when the view area displacement is corrected onlyin a specified direction as described above. Most of the patterns on asemiconductor wafer are formed in linear shapes, and the line widthseverywhere on a single line pattern are almost the same. Therefore, themeasurement of length can be accurately performed unless thedisplacement occurs only in the direction perpendicular to the pattern.

In a case where an objective image is a line pattern, and there is aview area displacement shown in FIG. 20( a) between two frames of theimages to be added, the relationship between shifting amount and degreeof agreement becomes as shown in FIG. 20( b). Referring to FIG. 20( b),blur in the added image is corrected by overlapping the images under acondition of maximizing the degree of agreement, but the condition ofmaximizing the degree of agreement exists not only at one position, butat positions distributed in a line shape. Accordingly, the conditionoverlapping the images (the condition of maximizing degree of agreement)can not determine uniquely. Therefore, since the blur of the pattern canbe corrected with the minimum shifting amount between the images whenthe shifting direction of the image is selected in the directionperpendicular to the pattern, there is an effect in that the effectiveview area of the added image is maximized.

EMBODIMENT 10

FIG. 14 shows a method of detecting the amount of positionaldisplacement and adding the corrected images. In an input image 1401 andan input image 1402, a region 1403 having an adequate size is put, forexample, in the central portion of the input image 1401, and templatematching is performed to the input image 1402 using the area 1403 as atemplate. Assuming that a region 1404 matches with the region 1403 asthe result, the region 1403 and the region 1404 are overlapped on eachother, and a rectangular region (an AND region) 1405 of overlapping theinput image 1401 and the input image 1402 on each other is set, and aportion not overlapping with the AND region in each of the input image1401 and the input image 1402 is cut off to form apost-position-adjusting input image 1406 or 1407, respectively. Addingprocessing is performed by inputting the post-position-adjusting inputimages 1406 and 1407.

This example shows the case of two input images, but it is easy toextend to a case of three or more input images. As an example of thetemplate matching, there is a method of executing normalized correlationprocessing between two images based on the following equation, where thesize of the input image is assumed to be 512×512 pixels and the size ofthe template in the center is assumed to be 256×256 pixels. Therein, aposition where the calculated correlation value becomes the maximum isdefined as a matching position.

${{r\left( {x,y} \right)} = \frac{\left\lbrack {{N{\sum\limits_{i,j}{P_{ij}M_{ij}}}} - {\left( {\sum\limits_{i,j}P_{ij}} \right)\left( {\sum\limits_{i,j}M_{ij}} \right)}} \right\rbrack}{\sqrt{\left\lbrack {{N{\sum\limits_{i,j}P_{ij}^{2}}} - \left( {\sum\limits_{i,j}P_{ij}} \right)^{2}} \right\rbrack\left\lbrack {{N{\sum\limits_{i,j}M_{ij}^{2}}} - \left( {\sum\limits_{i,j}M_{ij}} \right)^{2}} \right\rbrack}}},$Therein, r(x, y) is a correlation value at (x, y), M_(ij) is a densityvalue at a point (i, j) inside the template, P_(ij) is a density valueat a corresponding point (x+1, y+1) of the input image, and N is numberof pixels of the template.

FIG. 15 shows another embodiment of a method of adding corrected images.Similarly to FIG. 14, in an input image 1401 and an input image 1402, aregion 1403 having an adequate size is put, for example, in the centralportion of the input image 1401, and template matching is performed tothe input image 1402 using the area 1403 as a template. Assuming that aregion 1404 matches with the region 1403 as the result, the region 1403and the region 1404 are overlapped on each other, and a rectangularregion (an OR region) 1501 including both of the input image 1401 andthe input image 1402 is set, and a portion not overlapping with the ORregion in each of the input image 1401 and the input image 1402 isadded, and each of the added portions is filled with number of pixels of0 or an average value of each of the input images to form apost-position-adjusting input image 1502 or 1503, respectively. Addingprocessing is performed by inputting the post-position-adjusting inputimages 1502 and 1503. This example shows the case of two input images,but it is easy to extend to a case of three or more input images.

EMBODIMENT 11

Description will be made below on an example in which the driftcorrection technology in accordance with the present invention isapplied to automatic operation of a semiconductor inspection scanningelectron microscope. In general, in order to automatically operate thesemiconductor inspection scanning electron microscope, a recipe file towhich information such as measuring positions and observing conditionsis registered is formed in advance, and then measurement positioning,observation and measurement are performed according to the file. In thepresent method, an environment set before executing the recipe file isregistered. FIG. 19 shows a recipe execution environment page.

Main sequence of executing the recipe is as follows. That is, initially,alignment for detecting a position of a wafer on a stage is executed. Atthat time, image recognition is performed according to an imageregistered at forming the recipe. Next, the wafer is moved to themeasuring position using the stage, and an image is acquired with acomparatively low magnification. Positioning of the measured pattern(called as addressing) is performed with high accuracy by imagerecognition, and pattern dimension measurement is performed byelectrically deflecting the electron beam and zooming up to themeasuring magnification. Automatic focus adjustment is performed beforethe positioning of the pattern or before the measurement.

When test execution of the recipe or in a case where there are aplurality of measured wafers, an amount of drift during the time periodfrom acquiring of an image for positioning the pattern to acquiring animage for measurement is measured for each of the measured points usingthe first wafer. In the case of alignment, an amount of drift at severalminutes after the alignment is measured and stored. At executing therecipe, an amount of drift at each of the measured points or thealignment point is added to the image as an offset after positioning. Inthe case of addressing, the electron beam is deflected to a positionadded with the offset and the magnification is zoomed up to themeasuring magnification. By doing so, the drift after positioning can bereduced, and the plurality of samples can be measured with highthroughput because it is unnecessary to detect the amount of drift atthe actual measurement using the recipe or at measuring the secondwafers and wafers after the second. Whether or not the drift correctionis executed at alignment, at addressing or at measurement is judged byON or OFF of a drift correction switch 1901 for alignment, a driftcorrection switch 1902 for addressing or a drift correction switch 1903for measurement, respectively.

By providing the environment setting page to be described in the presentembodiment, it is possible to set a concrete method of drift correctionwhich changes depending on a measurement condition and a status of asample.

In recent manufacturing and inspection of semiconductors, a plurality ofsemiconductor wafers are usually dealt by the cassette unit bycontaining the semiconductor wafers in a cassette. An apparatus forcontinuously measuring such a plurality of measured objects is providedwith a means for selecting whether or not drift correction is performedbased on an amount of correction registered at forming the recipe and ameans for selecting whether or not drift correction is performed basedon an amount of drift actually measured each wafer. By constructed asdescribed above, when there is individual difference of thesemiconductor wafers in the cassette, the operator judges whether or notthe measurement accuracy takes precedence over the throughput, and theselection can be reflected to the measurement.

In a case of performing offset correction, a means for selecting whetheror not offset correction is performed based on a value registered atforming the recipe and a means for selecting whether or not offsetcorrection is performed using a value used for detecting the amount ofdrift in the first wafer in the cassette and registered are provided. Byconstructed as described above, when there is a manufacturing errorbetween a test pattern or a design value and an actual pattern, theoperator judges whether or not the measurement accuracy takes precedenceover the throughput, and the selection can be reflected to themeasurement.

Although the above description is the example in which the operatorselects the concrete correcting method, the present invention is notlimited to the above. For example, it is possible to provide a sequencewhich automatically sets the concrete method described above byinputting a magnitude of manufacturing error or presence ofmanufacturing error.

Although the above embodiments have been described on the cases of usingthe scanning electron microscope, the present invention is not limitedto the scanning electron microscope. The present invention can beapplied to a charged particle beam apparatus of another type in which asample image is displaced due to some drift producing cause.

1. A method of forming a sample image by scanning a charged particlebeam on a sample and forming an image based on secondary signals emittedfrom said sample, the method comprising the steps of: forming aplurality of composite images by superposing a plurality of imagesobtained by a plurality of scanning times; detecting a plurality ofpositional displacements between the composite images; and forming afurther composite image by correcting said positional displacementsamong said plurality of composite images based on the detectedpositional displacements and superposing said plurality of compositeimages.
 2. A method of forming a sample image by scanning a chargedparticle beam on a sample and forming an image based on secondarysignals emitted from said sample, the method comprising the steps of:two-dimensionally scanning said charged particle beam on said sample;detecting secondary signals emitted from a scanned region; forming imagedata based on said detected secondary signals; forming a plurality ofcomposite image data items formed by superposing said plurality of imagedata items; detecting a plurality of positional displacements in saidtwo-dimensional directions among said plurality of formed compositeimage data items; and forming an image based on the detected positionaldisplacements by correcting said detected positional displacements.
 3. Acharged particle beam apparatus comprising a charged particle source; adeflector for scanning a charged particle beam emitted from said chargedparticle source; and a detector for detecting secondary signals emittedfrom a scanned region of said charged particle beam, a sample imagebeing formed based on said secondary signals detected by said detector,which further comprises: a control unit which forms a plurality of imagedata items based on said detected secondary signals, and forms aplurality of composite image data items by superposing said plurality ofimage data items, and detects a plurality of displacements between thecomposite image data items, and corrects displacements among saidplurality of composite image data items based on the detecteddisplacements, and then superposes said plurality of composite imagedata items.
 4. A charged particle beam apparatus according to claim 3,wherein said plurality of composite image data items are formed by theunit of two-dimensional scanning performed by said deflector.
 5. Acharged particle beam apparatus according to claim 3, wherein saidplurality of composite image data items are acquired in different timingfrom one another.
 6. A charged particle beam apparatus according toclaim 3, which further comprises an image shift deflector for shifting ascanning region of said charged particle beam; and a sample stage forshifting said sample, wherein said control unit detects directions ofdisplacements among said images, and operates said image shift deflectorand/or said sample stage so that said scanning regions may be positionedtoward directions canceling said displacements.
 7. A charged particlebeam apparatus according to claim 6, wherein said control unit shiftssaid scanning region using said sample stage and/or said image shiftdeflector when an amount of said displacement is larger than a presetvalue, and shifts said scanning region using said image shift deflectorwhen an amount of said displacement is smaller than the preset value. 8.A charged particle beam apparatus according to claim 3, which furthercomprises an image sharpening means for sharpening said image to besuperposed, and a displacement between said image data items is detectedbased on image data sharpened by said image sharpening means.
 9. Acharged particle beam apparatus according to claim 3, wherein number ofpixels of said composite image data formed by superposing said pluralityof image data items is larger than number of pixels of the image databefore being superposed.
 10. A method of forming a sample image byscanning a charged particle beam on a sample and forming an image basedon secondary signals emitted from said sample, the method comprising thesteps of: two-dimensionally scanning said charged particle beam on saidsample; detecting secondary signals emitted from a scanned region;forming at least three images based on said detected secondary signals,wherein said images are obtained at different times by scanning saidcharged particle beam on said sample; by using at least two of saidthree images, detecting deformation or positional displacement in theother one of said three images; and correcting deformations ordisplacements of said image based on said detected deformation.
 11. Acharged particle beam apparatus comprising a charged particle source; adeflector for scanning a charged particle beam emitted from said chargedparticle source; and a detector for detecting secondary signals emittedfrom a scanned region of said charged particle beam, a sample imagebeing formed based on said secondary signals detected by said detector,which further comprises: a control unit for two-dimensionally scanningsaid charged particle beam on said sample; detecting secondary signalsemitted from a scanned region; forming at least three images based onsaid detected secondary signals, wherein said images are obtained atdifferent times by scanning said charged particle beam on said sample;using at least two of said three images, detecting deformation orpositional displacement of the other one sheet; and correctingdeformations or displacements of said image based on said detecteddeformation.
 12. An electron beam apparatus comprising a chargedparticle source; a deflector for scanning a charged particle beamemitted from said charged particle source; and a detector for detectingsecondary electrons emitted from a scanned region of said electron beam,which further comprises: a reflected electron detector for detectingreflected electrons emitted from said sample and/or an X-ray detectorfor detecting X-rays emitted from said sample; and an image display unitfor display a sample image based on reflected electrons detected by saidreflected electron detector or X-rays detected by said X-ray detector,wherein a shifting amount of a secondary electron image detected by saidsecondary electron detector is detected, and displacement of a view areabetween reflected electron images or X-ray images displayed on saidimage display unit is corrected based on shifting amount of thesecondary electron image.
 13. A method of forming a sample image byscanning a charged particle beam on a sample and forming an image basedon charged particles emitted from said sample, the method comprising thesteps of: scanning said charged particle beam on said sample pluraltimes; obtaining a plurality of images based on detection of secondarysignals emitted from said sample based on said plural times of scanning;removing images having abnormality from said plurality of images; andsuperposing said plurality of images excluding said images havingabnormality to form a composite image.
 14. A charged particle beamapparatus comprising a charged particle source; a deflector for scanninga charged particle beam emitted from said charged particle source; and adetector for detecting secondary signals emitted from a scanned regionof said charged particle beam, a sample image being formed based on saidsecondary signals detected by said detector, which further comprises: acontrol unit which forms a plurality of image data items based on saiddetected secondary signals, and removing image data items havingabnormality from said plurality of image data items, and superposingsaid plurality of images excluding said image data items havingabnormality.
 15. A method of forming a sample image by scanning acharged particle beam on a sample and forming an image based on chargedparticles emitted from said sample, the method comprising the steps of:selectively correcting displacements in a specific direction which isperpendicular to a line direction of a line pattern among a plurality ofimages obtained by plural times of scanning; and superposing saidplurality of corrected images to form a composite image.
 16. A method offorming a sample image according to claim 15, wherein said specificdirection is a direction intersecting at right angle with a longitudinaldirection of a line pattern formed on said sample.
 17. A chargedparticle beam apparatus comprising a charged particle source; adeflector for scanning a charged particle beam emitted from said chargedparticle source; and a detector for detecting secondary signals emittedfrom a scanned region of said charged particle beam, a sample imagebeing formed based on said secondary signals detected by said detector,wherein said detector is constructed to detect a plurality ofdisplacements based on said secondary signals, which further comprises:means for forming N0 frames of image data items based on said detectedsecondary signals, and for forming N1 frames of composite image dataitems by superposing said N0 frames of image data items; and settingmeans for setting said N0 and said N1.